1. Field of the Invention
The present invention relates to a process for producing a modified carbon black for rubber reinforcement (hereinafter sometimes simply referred to as xe2x80x9cmodified carbon blackxe2x80x9d). More specifically, the present invention relates to a process for producing a composite of carbon black and silica by using an organoalkoxysilane compound to produce a modified carbon black. The modified carbon black for rubber reinforcement thus obtained can be used in rubber compositions for tires and also for various types of rubber products such as rubber compositions for belt conveyors and industrial rolls, to provide rubber physical properties such as the decrease in tan xcex4 at a high temperature atmosphere (60xc2x0 C.-100xc2x0 C.), low heat generation due to the deformation and low loss of the driving energy. Especially, when the rubber composition containing the modified carbon black is used as a rubber composition for a tire tread, etc., tan xcex4 at around 60xc2x0 C. corresponding to the rolling resistance of the tire can be decreased, without decreasing tan xcex4 at around 0xc2x0 C. corresponding to braking performance of the tire, and therefore, tires having superior braking performance and low combustion cost can be produced. In addition, the modified carbon black of the present invention can be produced, without any major changes in the conventional carbon black production facilities, and therefore, can be produced at a low cost.
2. Description of the Related Art
Reinforcing fillers such as carbon black and silica have long been used for reinforcing rubber as is well known in the art. Since, among these fillers, silica, as compared with carbon black, has properties of providing a low tan xcex4 at high temperatures (i.e., around 60xc2x0 C.) and a high tan xcex4 at low temperatures (i.e., around 0xc2x0 C.), when used for rubber compositions for tire treads for example, a tire having a low rolling resistance and a high gripping power can be advantageously produced. However, silica is inferior to carbon black in the poor abrasion resistance and the low electrical conductivity, and therefore, if used for tires, there is the problem that the tire will pick up a charge when driven on and will cause various problems such as noise and, in some cases, even misoperation in electronic equipments such as radios.
Covering the surface of a pigment, etc. with silica etc. to improve the dispersion and to increase weather resistance has been proposed in, for example, Japanese Examined Patent Publication (Kokoku) No. 50-14254, Japanese Examined Patent Publication (Kokoku) No. 7-30269 and Japanese Unexamined Patent Publication (Kokai) No. 8-277347. For example, Japanese Examined Patent Publication (Kokoku) NO. 7-30269 discloses a method of treating the surface of carbon black used for a powder paint comprising the steps of dispersing carbon black in water, adjusting the pH to 6 or more, and, while maintaining the temperature of 70xc2x0 C. or more, precipitating amorphous silica on the surface of the particles of carbon black using sodium silicate. Further, Japanese Unexamined Patent Publication (Kokai) No. 8-277347 discloses the adhesion of silica to the surface of carbon black used for rubber reinforcement. However, there are still needs for modified carbon black for rubber reinforcement capable of improving the balance of tan xcex4 (0xc2x0 C.) and tan xcex4 (60xc2x0 C.) when compounded into rubber.
Accordingly, an object of the present invention is to provide a process for simply and cheaply producing a modified carbon black for reinforcement of rubber having a superior balance of tan xcex4 (0xc2x0 C.) and tan xcex4 (60xc2x0 C.), without losing the other characteristics.
In accordance with the present invention, there is provided a process for producing a modified carbon black for rubber reinforcement comprising:
adding, to carbon black, water-dispersed silica and an organosilane compound having the formula (I):
R1nxe2x80x94Sixe2x80x94(OR2)4-nxe2x80x83xe2x80x83(I) 
wherein R1 represents a hydrocarbon group having 1 to 10 carbon atoms, R2 represents a hydrogen atom or a hydrocarbon group having 1 to 10 carbon atoms, n is 1 or 2, and R1 and R2 may be the same or different, and
mixing and dispersing the mixture.
In accordance with the present invention, there is also provided a rubber composition comprising a crosslinkable rubber and the modified carbon black, compounded therein, produced by the above-mentioned process.
As the carbon black for rubber reinforcement used as the starting material for production of a modified carbon black for rubber reinforcement in the present invention, it is possible to use any carbon black which is generally used in the past for tires and other rubber compositions. Preferable carbon blacks are the SRF to SAF grades. It is possible to use the same differently or blend two or more types depending on the application of the rubber composition.
A general method for producing a modified carbon black for rubber reinforcement according to the present invention will be explained below. First, as the water-dispersed silica, it is possible to use those produced by mixing and reacting a metal silicate salt and an acid. The form of the water-dispersed silica (e.g., it may be either a sol or a gel) and the concentration of the silica in the water-dispersed silica are not particularly limited, but a sol type silica in which the silica particles exist independently is preferred. A silica concentration of 50,000 ppm or less, which is stable for a sol, is suitable. Further, as the water-dispersed silica, it is also possible to use a commercially available water-dispersed silica. As a commercially available water-dispersed slurry, those having a diameter of the silica particles contained of from 1 to 100 nm are preferred, more preferably those having a diameter of 5 to 80 nm, and having a sodium ion of 1.0% by weight or less, in terms of Na2O are preferred. This type water-dispersed silica is composed of primary particles or aggregates containing plural primary particles dispersed in water. Usually, the electrolyte content is extremely small, and therefore, it is stabilized in a basic state containing a high concentration of silica. Thus, the water-dispersed silica may be particularly suitably used when the silica content is intended to be increased in modified carbon. In the present invention, when producing a modified carbon black, the water-dispersed silica can be added, without particularly adjusting the pH thereof, but it is also possible, for example to add an acid to adjust the neutral conditions to prevent the gelation of the silica or to prevent the corrosion of the granulator. Thereafter, the silica is mixed with the carbon black, whereby the composite of the carbon black is formed.
According to the present invention, in the production of the carbon black for rubber reinforcement, the organosilane compound having the formula (I), in addition to the water-dispersed silica, is added. In the formula (I), the group R1 represents a hydrocarbon group having 1 to 10 carbon atoms, that is, a saturated or unsaturated aliphatic hydrocarbon group (i.e., typically alkane, alkene or alkyne group) having 1 to 10, preferably 1 to 6, carbon atoms, a alicyclic hydrocarbon group (i.e., typically cycloalkyl group) having 3 to 10, preferably 3 to 8, carbon atoms, or an aromatic hydrocarbon group (i.e., typically phenyl group) having 6 to 10, preferably 6 to 8, carbon atoms, and R2 represents a hydrogen atom or a hydrocarbon group similar to R1.
In the production of the modified carbon black for the rubber reinforcement according to the present invention, the total amount of the water-dispersed silica and the organosilane compound is preferably 0.1% to 50% by weight, more preferably 0.5% to 25% by weight, in terms of SiO2, based upon the total weight of the modified carbon black. Further, the amount of the organosilane compound is preferably 80% by weight or less, more preferably 1% to 50% by weight, in terms of SiO2, based upon the total amount of the water-dispersed silica and the organosilane compound. If the addition amount of the water-dispersed silica and the organosilane compound is too small, the reactivity with a silane coupling agent tends to decrease. Contrary to this, if the amount is too large, the electrical conductivity of the compounded rubber tends to increase. In addition, if the addition amount of the organosilane compound is too small, the adhesion power between the surface of the carbon black and the particles of colloidal silica tends to decrease. Contrary to this, if the amount is too large, the amount of the alcohol generated during the production is increased so that the treatment thereof becomes troublesome.
Note that the organosilane compounds having the formula (I) are known in the art and the commercially available products can be used directly. Examples thereof are 3-acetoxypropyltrimethoxysilane, 3-acryloxypropylmethyldimethoxysilane, 3-acryloxypropyltrimethoxysilane, allyltriethoxysilane, allyltrimethoxysilane, amyltriethoxysilane, benzyltriethoxysilane, 5-(bicycloheptenyl)triethoxysilane, t-butoxytrimethylsilane, n-butyltrimethoxysilane, [2-(3-cyclohexenyl)ethyl]trimethoxysilane, (3-cyclopentadienylpropyl)triethoxysilane, dicyclohexyldimethoxysilane, diethyldiethoxysilane, diethyl(triethoxysilylpropyl)malonate, (3,3-dimethoxypropyl)trimethoxysilane, dimethyldiacetoxysilane, dimethyldiethoxysilane, dimethyldimethoxysilane, diphenyldiethoxysilane, diphenyldimethoxysilane, ethyltriethoxysilane, ethyltrimethoxysilane, n-hexyltrimethoxysilane, isobutyltrimethoxysilane, 3-methoxypropyltrimethoxysilane, methyldiethoxysilane, methyldimethoxysilane, methylphenyldiethoxysilane, methylphenyldimethoxysilane, methyltriethoxysilane, methyltrimethoxysilane, methyltri-n-isopropoxysilane, octa-7-enyltrimethoxysilane, n-octylmethyldimethoxysilane, n-octyltriethoxysilane, phenetyltrimethoxysilane, phenyltriethoxysilane, phenyltrimethoxysilane, phenylvinyldiethoxysilane, n-propyltrimethoxysilane, styrylethyltrimethoxysilane, vinylmethyldiethoxysilane, vinyltriacetoxysilane, vinyltriethoxysilane, vinyltriisopropoxysilane, vinyltrimethoxysilane, vinyltriphenoxysilane, and vinyltriisopropenoxysilane.
The characteristic feature of the present invention resides in the fact that the water-dispersed silica and the organosilane compound are dispersed in carbon black using an internal type mixing device or a continuous type mixing device, followed by drying to produce the modified carbon black comprising carbon black having silica adhered to the surface thereof. Accordingly, the desired modified carbon black having silica adhered to the surface thereof can be produced by adding the water-dispersed silica and the organosilane compound at the previous step of, or before or during the granulating step (either continuous type or batch type) used for producing a normal or ordinary carbon black to produce a modified carbon black composed of carbon black having silica adhered to the surface thereof. Normally, water-dispersed silica is diluted with water, or an acid (e.g., sulfuric acid, hydrochloric acid) is added thereto to bring it to the neutral region, and then this is added to the carbon black. The carbon black added with the water-dispersed silica and the organosilane compound in the above way may be stirred, granulated, and modified at a suitable temperature (e.g., 60xc2x0 C. to 200xc2x0 C.) using, for example, a pin type screw granulator etc.
According to the present invention, the modified carbon black for rubber reinforcement can be compounded into any cross-linkable rubber to improve the tan 6 balance. Examples of such a cross-linkable rubber are natural rubber (NR), various butadiene rubbers (BR), various styrene-butadiene copolymers (SBR), polyisoprene rubber (IR), butyl rubber (IIR), halogenated butyl rubber, acrylonitryl-butadiene rubber, chloroprene rubber, ethylene-propylene copolymer rubber, ethylene-propylene-diene copolymer rubber, styrene-isoprene copolymer rubber, styrene-isoprene-butadiene copolymer rubber, isoprene-butadiene copolymer rubber, chlorosulfonated polyethylene, acryl rubber, epichlorohydrin rubber, polysulfide rubber, silicone rubber, fluororubber, urethane rubber. These may be used alone or in any blend thereof. When a blend is used, the ratio of the blend is not particularly limited.
The modified carbon black for rubber reinforcement according to the present invention is compounded into the above rubber component in an amount of, as a solid content, 10 to 200 parts by weight of the modified carbon black for rubber reinforcement, more preferably 15 to 150 parts by weight, based upon 100 parts by weight of the rubber component. If this amount is too small, sufficient reinforcement for the rubber is not possible and, for example, the abrasion resistance etc. deteriorate. Conversely, if the amount is too large, there is the danger of the hardness becoming too high, the processability falling, or the usefulness as a rubber material otherwise becoming poorer. It is also possible to use in the rubber composition any carbon black and/or silica normally formulated into rubber compositions, in addition to the above water-dispersed silica-modified carbon black.
The rubber composition may also have suitably formulated into it, in addition to the above rubber, modified carbon black having the silica adhered to or deposited on its surface, etc., any additives normally used in the rubber industry such as sulfur, organic peroxides, softening agents, antioxidants, vulcanization accelerators, fillers, plasticizers, silane coupling agents, etc., if necessary, in an ordinary used amount.